Hi,
I’m wondering if there is any Sonoff/Shelly alternative that uses zigbee instead of Wifi? The few ones I’ve found cost much more than Sonoff and Shelly. I really wonder why this is the case that there aren’t any good power switches available using zigbee which uses much less energy than Wifi.
I think the answer is that no one has built the zigbee equivalent of the esp8266.
You could build your own using an arduino and a xbee and a relay.
IKEA Tradfri line has a smart switch which has similar features as with Sonoff / Shelly.
There are also other types on the market. Just Google “zigbee smart switch”.
I am currently in the process of transitioning from WiFi (esp8266 based) devices and sensors to zigbee alternatives. Once you reach a certain number of WiFi devices … the access point / WiFi router looses performance, and that’s my main reason to go for zigbee.
Of course it could also be a sign to upgrade your wifi hardware 
It might just be cheaper to add an extra wifi AP.
This defeat the goal of extending the existing zigbee network, I’m also searching for something similar.
Xiaomi has switches similar to sonoff mini/ Shelly which use zigbee. They cost about 2 times as much as a Shelly though.
I have one of those, works perfectly. It’s a bit more expensive than a shelly (not quite double to get shipped to Australia) but still reasonably cheap.
And power consumption is higher as well in WiFi.
For me Xiaomi (Aqara) 2-channel zigbee relay is not an alternative. I tried it, but it works bad with wall mounted momentary switches (need to push very fast to make it work properly, otherwise it turns off right away — Shelly works perfectly).
P.S. I tried aqara with Berker 5035 and Berker 503404.